A complicated art, when performed correctly 'shissing' exemplifies multi-tasking and unequivocable prowess to the hilt of mankind's uppermost eschalons. Quite literally, 'to shiss' is to excrete simultaneously, which is too say 'pissing whilst shitting...shissing'.
A fundamental quality of military leaders and comedic performers alike, shissing nevertheless assumes the position of social taboo, and is thus confined to confidential conversation and, inevitably, urbandictionary.com
Person 1: Buddy, i have an issue...i need to do a twosie and onesie...at the same time
Person 2: Don't worry, just take a shiss
Person 1: Fo' shissle